You are absolutely correct.
Matter of fact, I always use at least the front sight in conjunction with a red dot and sit the dot on the tip top of the front post and this has always worked well for me since red dots and back up sights first came into being. I hardly ever use a rear sight at the same time as the optic (and front) because the sight picture can become crowded and busy, but as you said, if the optic poops out, the rear iron is there, ready,willing, and able.
Anyone who doesn't learn how to use a weapons' iron sights are doing themselves a huge disservice.